Radio-frequency (RF) mixers often use active voltage-to-current converters at the RF input. The transistors and resistors in the converters generate shot noise and thermal noise, respectively, which affect the signal-to-noise ratio (SNR) of the circuit. Shot noise power is proportional to the bias current of the converter. Therefore, to keep noise levels low, it is desirable to use low bias currents. On the other hand, bias currents are needed to shift the signals to levels in which conversion is approximately linear, i.e., in which a change in the input signal voltage results in a proportional change in the output current. When bias currents are too low, the signals are distorted. Consequently, setting the bias current implies a trade-off between noise and distortion.
Current approaches to overcome this problem include CMOS passive mixers, and inductive mixer degeneration. Both approaches, however, result in narrow-band mixers. CMOS mixers tend to be large to reduce transistor resistance, which increases their capacitance, thereby limiting bandwidth. In inductive degeneration, the inductors used resonate against the transistor capacitance likewise limiting bandwidth. Accordingly, there is a need for high-linearity, low noise converters that can be used over a wide bandwidth.